The benefits in speed and device density that can be obtained by reducing dimensions in integrated circuits (ICs), tend to be negated by increases in interconnect delay. This is the driving force behind the adoption of copper, which reduces resistivity, and lower-dielectric constant insulators, which reduce capacitance.
The formerly-used dielectric, SiO2 (silicon dioxide), has been replaced by SiOF (fluorine-doped silicon oxide), SiCOH (“carbon-doped oxide”—which adds organic groups such as —CH3 to the SiO2 network) and most recently porous SiCOH, also or pSiCOH. pSiCOH films may be deposited by liquid-phase or gas-phase methods.
SiOF and SiCOH reduce the dielectric constant of SiO2 by replacing some Si—O—Si linkages by Si—F or Si—CH3 (respectively). This introduces small voids (less than 1 nm in size) into the structure, reducing its overall polarizability and hence dielectric constant. As is well-known, the dielectric constants of air and most gases are very close to that of a vacuum, namely 1.0, so that introduction of voids reduces the overall dielectric constant below that of SiO2 (k=3.9). pSiCOH works in a similar fashion, introducing larger voids (approximately 1-5 nm) for further reduction in dielectric constant. This is typically accomplished by using a sacrificial “porogen” which is added during formation of the dielectric films and subsequently removed to form voids.
Although providing the desired reduction in dielectric constant, porous dielectrics are much more difficult to integrate into semiconductor manufacturing because their porosity causes a deterioration in mechanical properties. This creates problems in processes such as chemical mechanical polishing (CMP), during which low-k dielectrics can be easily cracked, and in packaging (i.e. final assembly ICs), where significant mechanical forces are applied to chips. In addition, the presence of pores allows reactive species to penetrate deep into the dielectric during subsequent processing steps, especially if the pores are interconnected. Reactive species can damage the low-k dielectric or contaminate it with undesired elements, such as metals.
One example of attack by reactive species is the conversion of Si—CH3 within the low-k film to Si—OH due to reaction with oxidizers. Si—OH groups are known to increase uptake of water into the low-k film. As liquid water has a dielectric constant of ˜80, even a small amount of water can lead to a dramatic increase in the dielectric constant. In addition, conversion of hydrophobic Si—CH3 groups to hydrophilic Si—OH leaves the film more vulnerable to attack by aqueous acids, which are used, for example, in cleaning steps. This can lead to an increase in the dimensions of a feature that is etched in a low-k dielectric film, then subsequently cleaned by dilute aqueous acids. As can easily be understood, any increase to the dimensions is very undesirable in the context of the overall drive to reduce feature size.
The difficulties of integrating low-k dielectrics into semiconductor manufacture have lead many manufacturers to avoid or postpone their implementation as long as possible. Comparison of the projections of the International Technology Roadmap for Semiconductors, 2007 edition, with actual manufacturing practice shows that low-k implementation is many years behind the schedule that was originally envisioned. As other solutions become exhausted, the need for implementation of low-k only grows.
There is therefore a need for improved low-k dielectrics that provide improved mechanical properties and improved resistance to reactive species.
pSiCOH films can be deposited by both gas and liquid phase methods. Although some liquid phase methods have entered into commercial production, the majority of films are deposited by gas phase methods, especially plasma-enhanced chemical vapor deposition (PECVD). PECVD is commercially successful largely because it uses equipment designs and know-how accumulated through years of experience on SiO2 and SiOF. Therefore it is particularly desirable to provide improved low-k dielectrics that can be deposited by PECVD.
Although less commercially successful, liquid-phase deposition methods do have certain advantages. Among those advantages is the relative ease of designing precursor molecules that incorporate desired properties into their molecular structure. As the liquid phase deposition process is relatively “gentle,” the structural characteristics of the molecule tend to be incorporated into the film. Under plasma conditions typically used for PECVD of low-k films, the plasma energy is sufficiently high that the structure of the precursor molecule is not preserved in the deposited film, but yet not high enough to eliminate the differences between films deposited using different precursors. Thus it is difficult to predict which precursor will deliver the desired film properties. Even apparently closely related precursors may result in very different films. Another aspect of PECVD is that it requires precursors that are sufficiently volatile for easy delivery to the deposition chamber.
Although it is widely believed that carbon linkages, i.e. replacing Si—O—Si with Si—[CH2]n—Si, improves mechanical properties of low-k films, identification of the most advantageous precursors has not been pursued to the extent necessary to predict the most effective solutions. While an understanding of this issue for films deposited from the liquid phase has been in place for some time—see for example an early review by Loy and Shea (Chem. Rev. 95 1431-1442 (1995))—similar understanding of films deposited by PECVD has yet to be developed. It is well known that films deposited by PECVD are much less ordered than can be achieved from the liquid phase and therefore their mechanical properties are not easy to predict.
The PECVD of pSiCOH films has been extensively studied and discussed by many authors and inventors. As summarized in, for example, U.S. Pat. No. 7,384,471 (“Vrtis”). The usual procedure is deposition of a hybrid film from an organosilicon precursor and an organic compound referred to as a “porogen”. The hybrid film is subsequently thermally treated, usually with accompanying exposure to ultraviolet light, to cause a major fraction of organic component to escape the film as gaseous species, forming pores. The thermal/uv treatment also increases cross-linking of the organosilane backbone, which improves mechanical properties. Alternative treatments such as e-beam curing, microwave curing and laser curing have also been extensively studied.
In contrast to non-porous SiCOH films (with k˜2.7 or greater), where several precursors have been successfully implemented, implementation of pSiCOH (at k=2.5 and lower) has been much more limited, and few precursors have been successful.
In U.S. Pat. No. 6,583,048, Vincent et al. argue that diethoxymethylsilane (DEMS) is a superior precursor for deposition of SiCOH films (without porogen), citing a Young's Modulus of 16.5 GPa achieved for a film with k=2.90. Vincent et al. cite other precursor examples which achieve lower Young's Moduli at comparable k, namely 8.76 GPa at k=2.85 for trimethylsilane and 6.68 GPa at k=2.88 for dimethyldimethoxysilane.
Vrtis presents Young's Moduli measurements for porous pSiCOH films deposited using DEMS and alpha terpinene (ATP). A modulus of 3.2 GPa was reported for a film with k=2.41. This value of Young's Modulus is substantially lower than is desirable for robust integration of the low-k film. Lower values of k were reported using di-t-butoxymethylsilane with ATP, but low Young's Moduli were again obtained (2.2 GPa at k=2.10 and 3.4 GPa at k=2.19).
Grill et al (U.S. Pat. No. 6,312,793) describe the use of cyclic organosilanes such as tetramethylcyclotetrasiloxane or alkylsilanes such as methylsilane with a porogen, such as bicyloheptadiene (BCHD).
Nguyen et al. (U.S. Pat. No. 7,491,658) describe the use of a single organosilicon precursor with a “built-in porogen” to deposit pSiCOH films. Examples of porogens with “built-in precursors” include vinyltriethoxysilane (VTEOS), vinylmethyldiethoxysilane (VMDEOS), and multiple others. Films with k in the 2.52 to 2.6 range were deposited, but measured Young's Moduli 2.94 to 3.78 GPa, again too low for robust integration.
Gates et al. (U.S. Pat. No. 7,288,292) describe deposition of low-k films using a combination of a cyclic siloxane precursor gas, a second precursor which is a porogen, and in some embodiments, another precursor comprising molecules that contain reactive groups sensitive to e-beam radiation. As further explained, the use of reactive groups sensitive to e-beam radiation is most advantageously coupled with an eventual curing step using e-beam radiation. Examples of groups sensitive to e-beam radiation that are listed include vinyl, allyl, phenyl, and acetylenic groups. Because the purpose of the third precursor is primarily sensitization to e-beam radiation, its concentration is limited to 0.1 to about 10% of the total precursor flow. The resulting films are said to be characterized by an elastic modulus of about 5 or greater (units assumed to be GPa) for k=2.4 or less and about 3 or greater for k=2.2 or less While both these ranges are described as “better than existing low-k films”, a Young's Modulus of 3 GPa is very low for practical integration.
Rhee et al. (U.S. Pat. No. 7,087,271) describe deposition of films using unsaturated organosilicon or organosilicate compounds or a combination of a saturated organosilicon or organosilicate compound with an unsaturated hydrocarbon. Combination of an unsaturated hydrocarbon with an unsaturated organosilicate is not discussed. Although dielectric constants of the resulting films were as low as 1.7 after annealing, mechanical performance was not discussed.
Sugahara et al. (U.S. Pat. No. 5,989,998) describe a method of forming an insulating film through plasma polymerization or oxidation of R1xSi(OR2)4−x where R1 is a phenyl group or a vinyl group and R2 is an alkyl group. Combination with a porogen is not addressed and the lowest dielectric constant reported for the trimethoxyphenylsilane compound alone is 3.0.
Wu et al. (U.S. Pat. No. 7,241,704) describe the deposition of pSiCOH films using a precursor and a porogen, either or both of which may contain a bulky organic group in order to create the desired porosity in the film. A dielectric constant of 2.2 was achieved, but no values are given for the Young's Modulus of the film.
Afzali-Ardakani et al (US Pat. App. Pub. No. 2008/0265381) describe the use of a porous dielectric in which all of the pores have a diameter of 5 nm or less, in order to achieve improved cohesive strength at lower dielectric constant. A very extensive list of candidate precursors and a large list of porogen precursors are provided. However, O'Neill et al. (Mater. Res. Symp. Proc. 914 2006) deposited multiple films having similar dielectric constant (in the range 2.46-2.53) using different porogen precursors with the same backbone precursor (DEMS). O'Neill et al. did not observe any correlation between film porosity and the molecular volume, degree of unsaturation or other characteristic parameter of the porogen. Therefore selection of the appropriate precursor and porogen from the lists provided by Afzali-Ardakani et al to achieve the desired cohesive strength and pore size is far from obvious.
A need remains for PECVD deposition methods using well-defined precursors that lead to low-k films with desired properties, such as low dielectric constant and high Young's Modulus. Although significant know-how exists regarding low-k film deposition and the desired properties of low-k films, precursor selection to provide the desired properties in a reliable manner has remained a challenge.